Protection of aluminum against atmospheric corrosion constitutes a challenge of significant economic importance. Several distinct aluminum alloys are known, characterized by different susceptibility to atmospheric corrosion. Among others, aluminum alloys containing a small percentage of Cu are well known and valued for their excellent mechanical properties, as, for example, Al 2024 T-3, widely applied in aircraft manufacturing industry.
It is well known, however, that due to copper rich intermetallic species randomly distributed in the aluminum matrix, which are spontaneously polarized as cathodic sites and catalyze the O2 reduction, the cathodic reaction of atmospheric corrosion, Al 2024 T-3 is also more susceptible to atmospheric corrosion.
There are two distinct corrosion control technologies commonly applied to protect aluminum alloys (such as Al 2024 T-3) against atmospheric corrosion: conversion coatings and organic coatings.
As for conversion coatings, Alodine 1200 is one of the well-known corrosion inhibitor technology widely applied for Al 2024 T-3 protection. It is based on soluble chromates containing CrO4−− as an inhibitor species and yields a robust conversion coating on aluminum substrates. A measure of its robustness, Alodine 1200 conversion coating on Al 2024 T-3 aluminum panels is known to resist salt spray exposure in excess of 300 hours, without pitting. In addition, conversion coatings are designed to enhance the adhesion of organic primers subsequently applied on aluminum substrates, a requirement also satisfied by Alodine 1200. Such procedures using chromates are thus considered to be the standard of the industry with respect to obtainable protection performance.
Aircraft primers and coil primers are the typical high performance organic coatings that are applied for protection of aluminum, such as especially in the aircraft manufacturing industry. A thickness of less than 20 micron is characteristic of these primers, which thus provide a negligible barrier function and, consequently, mandate the use of effective corrosion inhibitor pigments.
As is well known, pigment grade corrosion inhibitors used in organic primers must contain anionic species with inhibitor activity and must be characterized by limited, but effective, solubility in water. For these reasons, it will be apparent that CrO4−− is the corrosion inhibitor species preferred in both corrosion control technologies applied on aluminum for protection against atmospheric corrosion that is in conversion coatings and high performance organic primers.
SrCrO4 is the corrosion inhibitor pigment of choice for aircraft and coil primers, and is the standard in the industry. Due to environmental concerns, finding a replacement for chromates in conversion coatings and organic coatings constitutes the objective of contemporary research in this field.
It is generally known that if toxicity, efficiency, and price are considered, the number of inorganic corrosion inhibitor species available for chromate replacement is limited essentially to a few anionic species, and specifically to MoO4−−, PO4−−, BO2−−, SiO4−− and NCN−. As a consequence, all commercial non-chromate corrosion inhibitor pigments are molybdates, phosphates, borates, silicates or cyanamides, or combinations of these compounds. Except for Zn-(II) and Ce, which are credited with some degree of efficiency, the direct contribution of cationic species to the corrosion inhibitor performance of pigments is marginal. However, cations do determine the solubility and hydrolysis pH of pigments.
In comparison to CrO4−−, inherent limitations of their corrosion preventing mechanism render these above-specified anionic species less effective inhibitors of corrosion, in general, and specifically of atmospheric corrosion of aluminum. Consequently, it appears that inorganic chemistry is unable to produce inhibitors of atmospheric corrosion, which could be comparably effective, non-toxic alternative of CrO4−−. In contrast, a large arsenal of organic corrosion inhibitor is known and applied in various corrosion control technologies. Excessive solubility in water and/or volatility of most of the known organic inhibitors appear to be the physical properties that are inconsistent with applications in conversion coating technologies and in organic coatings. To date, no organic corrosion inhibitor is known to be an effective replacement of chromates in conversion coatings or organic coatings intended for metal protection.